Glucuronic acid is a novel source of pentosidine, associated with schizophrenia

Abstract

Pentosidine (PEN) is an advanced glycation end-product (AGEs), where a fluorescent cross-link is formed between lysine and arginine residues in proteins. Accumulation of PEN is associated with aging and various diseases. We previously reported that a subpopulation of patients with schizophrenia showed PEN accumulation in the blood, having severe clinical features. PEN is thought to be produced from glucose, fructose, pentoses, or ascorbate. However, patients with schizophrenia with high PEN levels present no elevation of these precursors of PEN in their blood. Therefore, the molecular mechanisms underlying PEN accumulation and the molecular pathogenesis of schizophrenia associated with PEN accumulation remain unclear. Here, we identified glucuronic acid (GlcA) as a novel precursor of PEN from the plasma of subjects with high PEN levels. We demonstrated that PEN can be generated from GlcA, both in vitro and in vivo. Furthermore, we found that GlcA was associated with the diagnosis of schizophrenia. Among patients with high PEN, the proportion of those who also have high GlcA is 25.6%. We also showed that Aldo-keto reductase (AKR) activity to degrade GlcA was decreased in patients with schizophrenia, and its activity was negatively correlated with GlcA levels in the plasma. This is the first report to show that PEN is generated from GlcA. In the future, this finding will contribute to understanding the molecular pathogenesis of not only schizophrenia but also other diseases with PEN accumulation.

Keywords: Advanced glycation end-products; Aldo-keto reductase; Glucuronic acid; Pentosidine; Schizophrenia.

Graphical abstract

Fig. 1

Differential plasma metabolites between subjects with high and normal pentosidine in cohort 1. Chemical structures of (A) PEN and (B) GlcA. (C) Volcano plot demonstrating the relationship between the mean rank difference in metabolites and the FDR (i.e., the q-value) of plasma metabolites. The high PEN group showed four upregulated and seven downregulated metabolites in the plasma compared to the normal PEN group. (D) Comparison of GlcA levels between high PEN and normal PEN groups. q = 5.40.E−05 using the Mann–Whitney test. (E) Correlation between GlcA and PEN in the plasma. Spearman’s correlation coefficient was r = 0.4111 (p < 0.0001). Comparison of known PEN-precursors levels between the high and normal PEN groups: (F) glucose, (G) fructose, (H) ribose, (I) arabinose, (J) xylose, and (K) ascorbate. Data are presented as the mean ± standard deviation (SD).

Fig. 2

Generation of pentosidine from glucuronic acid. (A) Amount of the yielded PEN from GlcA by incubating with lysine and arginine. (B) Chromatograms of standard PEN and the incubate samples with GlcA, lysine, and arginine (20 mM each) for one week. (C) Amount of the yielded PEN from GlcA and aldoses, other uronic acids, and precursor molecules of PEN by incubating with lysine and arginine. Two enlarged figures of the data are included at the top. (D) Amount of the yielded PEN from GlcA by incubating with BSA. Two-way ANOVA: F_{Interaction(4,16)} = 437, p < 0.001; F_Day(4,16) = 436, p < 0.001; F_GlcA(1,4) = 1296, p < 0.001. ***p < 0.001 (vs. control (−)) by Student’s t-test. (E) Staining with Coomassie brilliant blue and (F) western blotting with an anti-PEN antibody for BSA-PEN protein. (G) Amount of the yielded PEN from GlcA by incubating with human plasma. All the samples include plasma. Two-way ANOVA: F_{Interaction(2,6)} = 858, p < 0.001; F_GlcA(2,6) = 837, p < 0.001; F_{Incubation(1,3)} = 446, p < 0.001. ***p < 0.001 (vs. without incubation), ^###p < 0.001 (vs. 0 mM after incubation) by Tukey’s test. (H) Amount of the yielded PEN from GlcA and other precursor molecules of PEN by incubating with human plasma. One-way ANOVA: F_(4,12) = 154, p < 0.001. ***p < 0.001, **p < 0.01 (vs. control (−)), ^###p < 0.001, ^##p < 0.01 (vs. Glucose) and ^&&&p < 0.001 (GlcA vs. Ribose) using Tukey’s multiple comparison test. (I) Quantification of PEN in plasma of saline (SAL)- or GlcA-treated mice and guinea pigs. Two-way ANOVA: F_{Interaction(1,14)} = 24.6, p < 0.001; F_{Species(1,14)} = 110, p < 0.001; F_GlcA(1,14) = 27.8, p < 0.001. ***p < 0.001 (Mouse vs. Guinea pig with SAL), ^###p < 0.001 (SAL vs. GlcA in Guinea pig) by Tukey’s test. The data are shown as mean ± SEM values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Decreased AKR activity in schizophrenia. (A) Comparison of AKR activity in whole blood cells between control subjects (CON) and patients with schizophrenia (SCZ). *p < 0.05 by Mann–Whitney test. (B) Correlation between GlcA in plasma and AKR activity. Spearman’s correlation coefficient r = −0.1554, p = 0.0080.